Septins were discovered as mutants defective in cell division in budding yeast. Septins form filaments and compose a ring at the neck between mother and bud. Septins function as a diffusion barrier for mother / bud polarity and as a scaffold for regulatory proteins. We propose to study septin structure, assembly, and function in budding yeast. The septin ring contains filaments, seen by thin section EM and now in rotary shadowed preparations. The ring changes its shape and size through the cell cycle, and it has polarity. We will investigate filament and ring structure through the cell cycle, and the molecular mechanisms of their assembly and disassembly, using in vivo and in vitro approaches. The spindle position checkpoint ensures that the mitotic spindle moves into the mother / bud neck before exit from mitosis occurs, followed by cell division. Mitotic exit is controlled by the G-protein, Teml, which interacts with a GEF, Lte1, found only in the bud. We discovered septin mutants defective in restriction of Lte1 to the bud, consistent with a diffusion barrier role for septins. We will investigate the molecular mechanism of this effect. The spindle position checkpoint also appears to involve the interaction of cytoplasmic microtubules with a putative sensor protein at the neck. We hypothesize that the septin ring serves as the scaffold for the sensor, based on our published studies. To test this hypothesis, we identified new mutants defective in the spindle position checkpoint, and we will study how these genes contribute to the molecular mechanism of the spindle position checkpoint. In the bud morphogenesis checkpoint, the architecture of the septins is hypothesized to be the sensing mechanism. To test this hypothesis, we will examine septin filament architecture with respect to the cell cycle and checkpoint activation.